The present invention relates to a radio transmitting apparatus and a radio receiving apparatus used in a digital radio communication system.
With the recent spread of mobile communication apparatus such as portable telephones and pages, there has been an explosive increase in the number of users of such mobile communication apparatus. On the other hand, there is a limit to frequency resources available to radio communication, and therefore it is becoming extremely difficult to allocate a frequency band not used by existing radio systems when introducing a new radio communication system.
As new radio technology that enables effective use of frequency resources to deal with such a situation, an ultra wide band (UWB) transmission system has recently been drawing attention. Basically, the ultra wide band transmission system makes baseband transmission using a signal formed of a pulse train having a very small pulse width (for example 1 ns (nanosecond) or less). A bandwidth occupied by the ultra wide band transmission system is a bandwidth on the order of GHz such that a value obtained by dividing the occupied bandwidth by a central frequency (for example 1 GHz to 10 GHz) of the occupied bandwidth is substantially one. The bandwidth is extremely wide as compared with bandwidths used by the so-called W-CDMA system and cdma2000 system, and wireless LANs using SS (Spread Spectrum) and OFDM (Orthogonal Frequency Division Multiplexing).
In addition, the ultra wide band transmission system has a characteristic of low signal power density, and thus has an advantage in that the ultra wide band transmission method does not readily cause interference with another radio system. The ultra wide band transmission system is therefore expected to become a technology whose band can be overlaid on frequency bands used by existing radio systems. Furthermore, because of its wide band, the ultra wide band transmission system is considered to be promising as an ultrahigh-speed radio transmission technology at a level of 100 Mbps for application to a personal area network (PAN).
The ultra wide band (UWB) radio transmission has a characteristic of low signal power density, and thus has an advantage in that the ultra wide band radio transmission does not readily undergo or cause interference. Furthermore, because of its wide band, the ultra wide band radio transmission is considered to be promising as an ultrahigh-speed radio transmission technology at a level of 100 Mbps for application to a personal area network (PAN).
As a modulation system used in the ultra wide band transmission system, there is pulse position modulation (PPM) that uses a signal obtained by slightly shifting the timing of pulse generation forward and backward to express 0/1 information, as described in Japanese Translation of PCT for Patent No. Hei 10-508725 and U.S. Pat. No. 6,026,125, for example. In addition, as another modulation system, bi-phase modulation that expresses 0/1 information by changes in the phase of pulses has been proposed.
Supposing that a transmitting apparatus and a receiving apparatus of the ultra wide band transmission system using the above bi-phase modulation are formed using a pulse generator, the following configurations may be considered, for example.
FIG. 5 is a block diagram showing a configuration of a transmitting apparatus using such an ultra wide band transmission system. FIGS. 6A, 6B, 6C, 6D, and 6E are diagrams showing signal waveforms in parts of the transmitting apparatus shown in FIG. 5. FIGS. 7A, 7B, 7C, and 7D are diagrams showing signal spectra in the transmitting apparatus shown in FIG. 5.
A spread code sequence generator 502 generates a spread code sequence SG502 (FIG. 6B) with the frequency of a synthesizer 501, and then outputs the spread code sequence SG502 to a multiplier 503. The multiplier 503 multiplies a data sequence SG501 (FIG. 6A and FIG. 7A) by the spread code sequence SG502 to form a spread signal SG503 (FIG. 6C and FIG. 7B), and outputs the spread signal SG503 to an impulse generator 504.
The impulse generator 504 generates an impulse signal SG504 (FIG. 6D and FIG. 7C) of very fine impulses of 100 ps, for example, which signal corresponds to 0s/1s of the spread signal SG503. The impulse signal SG504 is outputted to a band-pass filter 505, where a predetermined range, for example a range of 3.0 to 5.0 GHz of the impulse signal SG504 is extracted. The impulse signal SG504 is thereby converted into a transmission signal SG505 (FIG. 6E and FIG. 7D), and then transmitted as SG505 via an antenna 506.
FIG. 8 is a block diagram showing a configuration of a receiving apparatus using the ultra wide band transmission system. FIG. 9 is a diagram showing correlation characteristics in main parts of a timing synchronization circuit having a configuration of a so-called DLL (Delay Lock Loop) in the receiving apparatus shown in FIG. 8.
A radio signal is received by an antenna 801. A band-pass filter 802 removes an undesired component from the received signal, and then outputs the resulting received signal to multipliers 803, 805, and 807.
A spread code sequence generator 811 generates a spread code sequence (spread code sequence identical with the spread code sequence used in the transmitting apparatus shown in FIG. 5) with the frequency of a synthesizer 815, and then outputs the spread code sequence to an impulse generator 812. The impulse generator 812 generates impulses, superimposes the spread code sequence outputted from the spread code sequence generator 811 on the impulses, and then outputs the result to delay circuits 813, 814 and the multiplier 807.
The delay circuit 814 delays the impulses having the spread code sequence superimposed thereon by a ½ pulse width, and then outputs the result to the multiplier 803. The delay circuit 813 delays the impulses having the spread code sequence superimposed thereon by one pulse width, and then outputs the result to the multiplier 805.
Hence, the multiplier 803 multiplies the received signal by the impulses having the spread code sequence superimposed thereon for demodulating transmitted data, to thereby perform despreading processing. The multiplier 807 multiplies the received signal by the impulses having the spread code sequence superimposed thereon in timing a ½ pulse width ahead of the output of the delay circuit 814, to thereby perform despreading processing. The multiplier 805 multiplies the received signal by the impulses having the spread code sequence superimposed thereon in timing a ½ pulse width behind the output of the delay circuit 814, to thereby perform despreading processing.
A result of the multiplication of the multiplier 803 is outputted to an integrator 804, integrated by the integrator 804, and then outputted as received data. A result of the multiplication of the multiplier 805 is outputted to an integrator 806, integrated by the integrator 806, and then outputted to a difference unit 809 (902 in FIG. 9). A result of the multiplication of the multiplier 807 is outputted to an integrator 808, integrated by the integrator 808, and then outputted to the difference unit 809 (901 in FIG. 9).
The difference unit 809 obtains a difference (903 in FIG. 9: solid line) between the output of the integrator 806 and the output of the integrator 808, and then outputs the difference to a loop filter 810. As is understood from FIG. 9, the output (axis of ordinates) linearly responds to phase displacement (axis of abscissas). Specifically, the output has a characteristic showing an S-shaped curve based on a reception timing offset.
Thus, an output (difference) obtained by filtering the difference by the loop filter 810 is fed back to the synthesizer 815. When the characteristic shown in FIG. 9 indicates no reception timing offset, for example, a zero is outputted; when reception timing is offset forward or backward, a positive or negative value is outputted as a timing offset signal. Such a circuit is referred to as a timing synchronization circuit (DLL: Delay Lock Loop).
The synthesizer 815 effects control such that when the output of the loop filter 810 is positive, the phase of the spread code sequence being generated is delayed slightly, and when the output of the loop filter 810 is negative, the phase of the spread code sequence being generated is advanced slightly. Thus, the output (difference) of the loop filter 810 becomes zero, and the spread code sequence and pulses having the received signal superimposed thereon supplied to the multiplier 803 are brought in phase with each other, whereby the despread output of the multiplier 803 is maximized.
However, the ultra wide band (UWB) radio transmission uses a frequency band of 3 GHz to 6 GHz. Although the ultra wide band (UWB) radio transmission does not use a frequency band of 2 GHz and lower, which band is used by many other systems, a wireless LAN system such as IEEE 802.11a uses 5 GHz. Even in an application to PAN, the frequencies used by the ultra wide band (UWB) radio transmission and the wireless LAN system conflict with each other in the same area, and therefore the ultra wide band (UWB) radio transmission and the wireless LAN system cause interference with each other or undergo interference from each other.
Because of its ultra wide band, the UWB radio transmission undergoes a considerably low level of interference. Supposing that the other system is used in substantially the same area, when a transmitter of the 5-GHz wireless LAN system is situated in the close vicinity of a UWB receiver or when a UWB transmitter is situated in the close vicinity of a receiver of the 5-GHz wireless LAN system, the level of interference may exceed a tolerable range, thereby greatly degrade receiving quality and render reception impossible.
As a method for solving this problem, there is WO01/93445 (applicant: XTREMESPECTRUM). According to this invention, when presence of another narrow-band system (ex. 5 GHz wireless LAN) is detected, the band is removed by using a filter circuit. However, the method has a problem in that the structure of the removal filter is complex. In addition, only the structure of a receiver for removing interference received from another system is disclosed, and no consideration is given from a viewpoint of preventing the causing of interference with the other system.